The invention relates to sensing devices incorporating single crystal semiconductor wafers.
Sensing devices such as silicon vidicons and silicon intensifier tubes employ sensing elements or targets consisting of single crystal semiconductor wafers. The operation of such sensing elements in these devices is well known in the art. One problem associated with their operation is that certain elemental signal areas of the sensing elements may become overly excited by an incident image signal. As a consequence of such overexcitation, charge carriers in excess of the signal handling capabilities of the wafer may be generated at certain localized regions of the sensing element or wafer. The excess charge carriers diffuse laterally to adjacent regions of the wafer, causing a loss of imaging capability in the neighboring regions of the wafer as evidenced by an undesired "blooming" or overloading of the target in the localized region.
One technique for controlling blooming is described in "Theory, Design, and Performance of Low-Blooming Silicon Diode Array Imaging Targets" by B. M. Singer and J. Kostelec in IEEE Trans. on Electron Devices, vol. ED-21, pp. 84-89, January 1974, herein incorporated by reference. In this technique, a wafer is proposed having a controlled energy level configuration and surface recombination velocity wherein a potential barrier is incorporated into the wafer at an unstated depth from its major input signal sensing surface. The doping level controls the height of the potential barrier. This potential barrier in normal operation allows a limited number of excited minority carriers to penetrate to the input signal sensing surface and then recombine, thereby maximizing the sensitivity of the device by permitting the greater majority of excited minority carriers to diffuse toward a charge storage region of the wafer along a major surface opposite the sensing surface of that wafer. However, in the case of the generation of excess carriers by over-excitation at localized regions (normally associated with the blooming condition previously described) the excess carriers accumulate at and overcome the potential barrier. These excess carriers are swept to the sensing surface where they quickly recombine due to the substantially increased recombination velocity along that surface, thereby avoiding lateral diffusion to other neighboring regions of the target.
Ideally, the energy level configuration (and the potential barrier) necessary for accomplishing the noted blooming reduction mechanism can be controlled by carefully depositing a fixed number of donors or acceptors to a specified depth of the sensing element or wafer by suitable techniques such as, for example, ion implantation. However, in order to maximize the sensitivity of imaging devices such as, for example, silicon vidicons, it is also necessary to locate the potential barrier as close to the major input signal sensing surface as possible. The sensitivity of the device to strongly absorbed input signals, such as blue or ultraviolet light, in particular, is reduced in accordance with the distance of the potential barrier to that surface. Unfortunately, in practical devices, certain processing methods, and variables associated therewith, in the manufacture of the sensing element or wafers requires that the barrier potential be located sufficiently distant (on the order of 3000 A) from the sensing surface to isolate or minimize the effect of those methods upon the barrier potential necessary for controlling blooming. Otherwise, it has been found that undesirable levels of dark current, and inadequate blooming control occurs. Undesirable and uncontrollable variations or instabilities also occur in the dark current and blooming control performance during manufacture or assembly. Furthermore, noncontrollable variations in processing reduce the manufacturing yield of useable wafers having the desired non-blooming characteristics to uneconomical levels. Such problems notwithstanding, it is desirable to provide controllable potential barriers of the type described at distances of less than about 1500 A from major input signal sensing surfaces to maximize the sensitivity characteristics of the ultimate device and yet provide blooming control with low dark current.